plone-compiled.css has a font-face declaration with src declarations along these lines: src:url('/../++plone++static/fonts/plone-fontello.eot') This will try to find ++plone++static on the root of the domain but we normally have sites on some path, e.g. foobar.com/plone as the site root.
I don't actually notice any odd behavior on sites - for instance the toolbar icons, which I think are this fontello font, still appear. It looks like it may be using an embedded font instead?